1. Field of the Invention
The present invention relates generally to methods and apparatus for ultrasonically imaging cased wells. More specifically, the invention relates to methods and apparatus for imaging and characterizing, with azimuthal resolution, (i) the annular region between the casing and the earth formations surrounding the casing and (ii) the wall surface of such earth formations.
2. Description of the Prior Art
In a well completion, a string of casing or pipe is set in a wellbore, and a fill material (typically cement) is forced into the annulus between the casing and the earth formations. The primary purpose of such cement is to separate oil- and gas-producing layers from each other, and from water-bearing strata.
If the cement fails to provide isolation of one zone from another, fluids under pressure may migrate from one zone to another, reducing production efficiency. In particular, migration of water into a hydrocarbon-bearing zone can, in some circumstances, render a well non-commercial. Also, migration of hydrocarbons into aquifers is environmentally and economically undesirable. Thus, imaging the annulus content, and, in particular, detecting interfaces between cement and a fluid-channel and/or between cement and the formation, is important for reliable determination of the hydraulic isolation of the different strata of a formation.
Current open-hole logging proceduresxe2x80x94using electrical devices, such as Schlumberger""s Fullbore Formation MicroImager (FMI), or acoustic devices, such as Schlumberger""s Ultrasonic Borehole Imager (UBI)xe2x80x94stress the importance of imaging the formation wall. These imaging techniques allow for identification of hydrocarbon-bearing beds within the earth formations, and for detection of fractures, breakouts, and washouts, to help assess well stability; however, they do not work through casing.
It is widely reported that a significant percentage of existing cased wells were never imaged prior to encasement. There may be several reasons why such wells were never imaged prior to encasement, e.g., lack of adequate imaging technology, cost, etc. Today, however, imaging of existing cased wells can be desirable for, among other things, detection and identification of so-called bypassed pay (i.e., hydrocarbon-bearing) zones.
Another need for through-the-casing imaging exists in the process of hydraulic fracturing, which typically takes place after a well has been cased, and is used to stimulate the well for production. Often, the fracturing process is accompanied by sanding, whereby certain strata of the formation release fine sand that flows through casing perforations into the well, and then up to the surface, where it can damage production equipment. This problem can be remedied if the sand-producing zones are detectedxe2x80x94as could be done, for example, with an imaging technology capable of operating through the casing.
Generally speaking, a cased well includes a number of interfaces at the junctures of the differing materials within the wellbore. A xe2x80x9cfirst interfacexe2x80x9d exists at the juncture of the borehole fluid in the casing and the casing. (The casing is generally referred to as a xe2x80x9cfirst materialxe2x80x9d and is typically comprised of steel.) A xe2x80x9csecond interfacexe2x80x9d is formed between the casing and a second material adjacent to the exterior of the casing. If cement is properly placed in the annulus, the xe2x80x9csecond interfacexe2x80x9d exists between the casing (i.e., the first material) and the cement (i.e., the second material). A xe2x80x9cthird interfacexe2x80x9d also exists between the cement and a xe2x80x9cthird materialxe2x80x9d (i.e., the formation).
The problem of investigating the fill material within the annulus has motivated a variety of cement evaluation techniques using acoustic energy. These techniques generally fall into two classes: (i) sonic cement evaluation and (ii) ultrasonic cement evaluation.
One sonic cement evaluation technique, described in U.S. Pat. No. 3,401,773, to Synott, et al., uses a logging tool employing a conventional, longitudinally spaced sonic transmitter and receiver. The received signal is processed to extract the portion affected by the presence or absence of cement. The extracted portion is then analyzed to provide a measurement of its energy, as an indication of the presence or absence of cement outside the casing. This technique provides useful information about cement detects at the second interface. However, sonic techniques have several limitations, such as: (i) poor azimuthal and axial resolutions, and (ii) strong sensitivity to the bond quality between the casing and the cement, thus requiring, in the cases of poor bond quality, internal pressurization of the casing, which, itself, can degrade cement integrity.
Ultrasonic cement evaluation tools, such as Schlumberger""s Cement Evaluation Tool (CET) and UltraSonic Imager (USI) Tool, concentrate on the second interface to determine whether cement or mud is adjacent to the casing. in the annulus between the casing and the earth formation. The measurement obtained by these tools is based on a pulse echo technique, whereby a single transducer, pulsed with a broad-band signal (ie., 200-600 kHz), insonifies the casing at near-normal incidence, and receives reflected echoes. The method of measurement is based on exciting a casing resonance, measuring the temporal period and amplitude decay rate, and interpreting the data to determine whether cement or undisplaced mud lies adjacent to the casing. Such ultrasonic techniques, optimized to yield information about casing thickness, are described in U.S. Pat. No. 2,538,114 to Mason and U.S. Pat. No. 4,255,798 to Havira. The main limitation of these pulse echo techniques is that little of the acoustic energy (i.e., typically less than 10 percent) is transmitted through the casing to probe the annulus.
U.S. Pat. No. 5,011,676 to Broding purports to address the problem of primary and multiple reflections from a well casing interfering with formation reflection signals. Broding suggests elimination of interfering casing reflections by using one or several acoustic transducers directed on the casing at incident angles that fall between the compressional and shear critical angles of (borehole fluid)-steel interface. such that only shear waves are excited within the casing and no compressional waves propagate therein. The method relies on the fact that no signal is received so long as the cement-casing interface is regular, the annulus contains no channels or discontinuities, and the cement-formation interface is also smooth. Hence, when a signal is received by the transducer, one or more than one of these conditions is violated. And Broding does not offer a methodology on how to relate the received signal to the scatterer responsible for establishing it. Moreover, the Broding disclosure also states that when the transducer energy is directed at an angle larger than the shear critical angle, no energy is transmitted through the casing and into the annulus. Applicant herein has found this to be incorrect.
EP 0549 419 B1 to Stanke et al. disclose a method and apparatus to determine hydraulic isolation of oilfield casings by considering the entire volume of the annulus between the casing and the earth formation, and characterizing the third interface formed at the juncture of a second material, contacting the outside of the casing, and a third material adjacent to, and outside, the second material. Interrogation of the xe2x80x9cthird interfacexe2x80x9d is performed by directing an acoustic pulse at a segment of the casing. Ultrasonic transducers aligned along the casing axisxe2x80x94at angles of incidence, with respect to the casing inner wall, falling within the compressional and shear critical angles of a water-steel interface, i.e., about 14 to 27 degreesxe2x80x94such that shear signals within the casing are optimized and compressional signals within the casing are excluded. To effectively track a third-interface echo as the annulus thickness varies, a receiver array and complex signal processing algorithm are required. Additionally, the measurement would be adversely affected in attenuative muds.
In light of the above, one object of the present invention is a method and apparatus for ultrasonically imaging cased wells which overcomes one or more of the above-noted disadvantages of traditional approaches.
Generally speaking, and without intending to be limiting, one aspect of the invention relates to method(s) for analyzing, imaging, or characterizing cased wells, such as, for example, a method comprising the following: (i) exciting a flexural wave in a casing by insonifying the casing with a pulsed, collimated acoustic excitation aligned at an angle greater than the shear critical angle of the fluid-casing interface; (ii) receiving one or more echoes, (iii) analyzing the echoes to characterize the cased well; and, optionally, (iv) providing an image of at least a portion of the cased well. Additionally, the invention may also involve: (iv) identifying a scatterer in the annulus or a feature of the formation wall; (v) utilizing an inversion method to derive a footprint for a probing beam directed toward said scatterer or feature; and (vi) utilizing information from (v) to obtain more accurate information about the size of said scatterer or feature. Still further, the invention may also involve: (iv) providing a 3D image of scatterers in the annulus and/or features of the formation wall and (v) zooming the 3D image in accordance with instructions from a user; or (iv) providing a 3D image of scatterers in the annulus and/or features of the formation wall and (v) reforming the 3D image to focus on a specific region of 3D space.
Moreover, in accordance with another general aspect of the invention, and, again, not intending to be limiting, xe2x80x9canalyzing the echoesxe2x80x9d may include one or more of the following: (a) analyzing the propagation time of the echoes to determine the location of scatterers within the annulus; (b) analyzing the amplitude of envelopes of the echoes to determine an approximation of the azimuthal and axial size of scatterers within the annulus; (c) analyzing the positive and/or negative peak amplitudes of echoes to determine the impedance of scatterers within the annulus; (d) determining whether the scatterers are fluid-filled channels or gas-filled channels; (e) analyzing the propagation time of echoes from the formation wall to determine hole diameter; (f) analyzing the propagation time of echoes from the formation wall to determine casing eccentricity; (g) analyzing the propagation time of echoes from the formation wall to determine wave speeds in the cement, and analyzing such wave,speed information to obtain information about cement mechanical properties; (h) analyzing the amplitude of echoes from the formation wall to detect and/or identify fractures and/or faults intersecting the borehole; (i) analyzing the amplitude and propagation time of echoes from the formation wall to detect and/or identify enlargements of the borehole diameter associated with breakouts, washouts, and/or cavities; (j) analyzing the positive and/or negative peak amplitude of echoes from the formation wall to detect and/or identify dipping beds in the formation; (k) utilizing an inversion method in which information about early-arriving echoes due to propagation in the casing is used to approximate the profiles of the energy transmitted into the annulus; (l) using these profiles to construct the profile of a probing beam that gives rise to the echoes originating in the annulus and at the formation wall, and utilizing an inversion method in conjunction with the profile of the probing beam to extract from the amplitude of the third-interface echo(es) the size of scatter(s) in the annulus and/or fractures on the formation wall; (m) analyzing early-arriving echoes to qualitatively evaluate the casing for corrosion and/or perforations; (n) analyzing early-arriving echoes to detect the presence of gas-like material at the casing-cement interface; (o) determining whether the early-arriving echoes resemble a time elongated wave-train (e.g., a ringing echo) and, if so, indicating the presence of gas-like material at the casing-cement interface; (p) analyzing the echoes to provide a qualitative indication of cement strength; (q) analyzing early-arriving echoes to extract their dispersion characteristics; (r) determining casing thickness from the dispersion characteristics; (s) determining casing metal loss from said casing thickness information; (t) processing echoes arriving after the early-arriving echoes to determine their multiplicity for qualitative determination of cement strength; (u) processing echoes arriving after the early-arriving echoes to determine their propagation time inside the cement; and/or, (v) processing echoes arriving after the early-arriving echoes to determine whether they arose from scatterers in the annulus or at the formation wall.
Again, generally speaking, and without intending to be limiting, flexural wave excitation may be achieved by insonifying the casing with a pulsed, collimated acoustic excitation aligned at an angle greater than the shear critical angle of the fluid-casing interface (about 25-29 degrees), or by any other method of creating a substantial, predominantly, or prevailingly flexural excitation in the casing.
Yet again, generally speaking, and without intending to be limiting, a still further aspect of the invention relates to an apparatus for inspecting, imaging, analyzing, or characterizing cased wells, such as, for example, an apparatus comprising the following: (i) means (of any type whatsoever) for exciting a flexural wave in a casing by insonifying the casing with a pulsed, collimated acoustic excitation aligned at an angle greater than the shear critical angle of the fluid-casing interface; (ii) means (of any type) for receiving one or more echoes; (iii) means (of any type) for analyzing the echoes to characterize the cased well; and, optionally, (iv) means (of any type) for providing an image of at least a portion of the cased well; or, optionally, (iv) means (of any type) for identifying a scatterer in the annulus or a feature of the formation wall; (v) means (of any type) for utilizing an inversion method to derive a footprint for a probing beam directed toward said scatterer or feature; and, (vi) means (of any type) for utilizing information from (v) to obtain more accurate information about the size of said scatterer or feature; or, optionally, (iv) means (of any type) for providing a 3D image of scatterers in the annulus and/or features of the formation wall; and, (v) means (of any type) for zooming the 3D image in accordance with instructions from a user; or, optionally, (iv) means (of any type) for providing a 3D image of scatterers in the annulus and/or features of the formation wall; and, (v) means (of any type) for reforming said 3D image to focus on a specific region of 3D space.
Furthermore, in accordance with still further aspects of the invention, and, again, not intending to be limiting, xe2x80x9cmeans for analyzing the echoesxe2x80x9d may include one or more of the following: (a) means (of any type) for analyzing the propagation time of the echoes to determine the location of scatterers within the annulus; (b) means (of any type) for analyzing the amplitude of envelopes of the echoes to determine an approximation of the azimuthal and axial size of scatterers within the annulus; (c) means (of any type) for analyzing the positive and/or negative peak amplitudes of echoes to determine the impedance of scatterers within the annulus (d) means (of any type) for determining whether the scatterers are fluid-filled channels or gas-filled channels; (e) means (of any type) for analyzing the propagation time of echoes from the formation wall to determine hole diameter; (f) means (of any type) for analyzing the propagation time of echoes from the formation wall to determine casing eccentricity; (g) means (of any type) for analyzing the propagation time of echoes from the formation wall to determine wave speeds in the cement and for using such information to compute information about mechanical properties of the cement; (h) means (of any type) for analyzing the amplitude of echoes from the formation wall to detect and/or identify fractures and/or faults intersecting the borehole; (i) means (of any type) for analyzing the amplitude and propagation time of echoes from the formation wall to detect and/or identify enlargements of the borehole diameter associated with breakouts, washouts, and/or cavities; (j) means (of any type) for analyzing the positive and/or negative peak amplitude of echoes from the formation wall to detect and/or identify dipping beds in the formation; (k) means (of any type) for utilizing an inversion method in which information about early-arriving echoes due to propagation in the casing is used to approximate the profiles of the energy transmitted into the annulus; (l) means (of any type) for using the profiles to construct the profile of a probing beam that gives rise to the echoes originating in the annulus and at the formation wall; (m) means (of any type) for analyzing early-arriving echoes to qualitatively evaluate the casing for corrosion and/or perforations; (n) means (of any type) for analyzing early-arriving echoes to detect the presence of gas-like material at the casing-cement interface; (o) means (of any type) for determining whether the early-arriving echoes resemble a time-elongated wave-train and, if so, indicating the presence of gas-like material at the casing-cement interface; (p) means (of any type) for analyzing the echoes to provide a qualitative indication of cement strength; (q) means (of any type) for analyzing early-arriving echoes to extract their dispersion characteristics; (r) means (of any type) for determining casing thickness from the dispersion characteristics; (s) means (of any type) for determining casing metal loss from said casing thickness information; (t) means (of any type) for processing echoes arriving after the early-arriving echoes to determine their multiplicity for qualitative determination of cement strength; (u) means (of any type) for processing echoes arriving after the early-arriving echoes to determine their propagation time inside the cement; and/or, (v) means (of any type) for processing echoes arriving after the early-arriving echoes to determine whether they arose from scatterers in the annulus or at the formation wall.
The above-referred to excitations may be created by single or multiple transmitting elements Similarly the above-referred to echoes may be received by single or multiple receiving elements.
The invention is preferably practiced using a combined apparatus (such as a sonde, or drill-string section) containing at least one excitation device and one receiving device. Such combined apparatus may be disposed (and vertically positioned) in a borehole by a wireline, by coiled tubing, as part of a drill-string, or by a robotic apparatus, and is preferably rotatable about the axis of the borehole to provide azimuthal information. Alternatively, azimuthal information may be obtained via a plurality of transmitter(s) and/or receiver(s) positioned concentrically about the axis of the borehole.